This study involves an investigation of the pharmacological and neuroendocrine regulation of pituitary hormone secretion from the normal hypophysis and adenomatous tissue. We are studying the dopaminergic mechanisms which regulate prolactin release from these tissues because the normal tissue, as contrasted with the tumor, responds to dopaminergic inhibition. A study of the dopamine receptor activity indicates surprisingly that both tissues contain very similar receptors; therefore, the reasons for the refractoriness of the tumors to dopamine inhibition must be related to a post-receptor deficit. We will investigate this possibility. These results have been obtained with MtTW15 and 7315a tumors which have been transplanted into rats for more than 25 years. We will produce other pituitary tumors through the administration of estrogen, determine whether these tumors are sensitive to dopamine and characterize the dopamine receptors therein.